Process of solid phase bonding of metals



11 Patent PROCESS OF SOLID BONDING OF METALS Sidney Siegel, Sharon, Mass., assignor to Metals & Controls Corporation, Attleboro, Mass., a corporation of Massachusetts 7 No Drawing. Application October 17, 1951, Serial No. 251,813 r 4 Claims. (Cl. 148-115) 1951, and issued as U. S. Patent No. 2,691,815, there is disclosed a method of solid phase bonding metals involving the steps of (1) meticulously preparing the mating surfaces of the metals to eliminate from substantiallythe entire area of said surfaces any film which Will act as a barrier to bonding; (2) squeezing the mating surfaces together (while still free from barrier film) with concomitant increase in area thereof to such an extent that at numerous isolated spots atoms of one metal come within the field of atomic attraction of the other, forming what are referred to as nucleal bonds; and (3) heating or sintering the nucleally bonded stock whereby the nucleal bonds are caused to grow laterally to increase the strength of the bond. The squeezing operation is carried out at cold working temperature, i. e., any temperature from room temperature up to the recrystallization temperature of the metal having the lowest recrystallization temperature.

In the coassigned copending application of Helmieh W. Boessenkool, George Durst and 'Sidney Siegel entitled Solid Phase Bonding of Metals, Serial No. 204,646, filed January 5, 1951, there is disclosed a method of solid phase bonding metals within an elevated temperature range, extending from just above the minimum recrystallization temperature of the metal having the lowest recrystallization temperature up to the highest temperature at which the metals may be deformed without any pulling apart or any formation of brittle compounds or liquid phase material at the interface of the metals being bonded. Bonding is accomplished by heating at least one of the metals in a protective atmosphere to a temperature within the said elevated temperature range, squeezing the metals together with concomitant increase in area thereof to such an extent that at numerous isolated spots atoms of one metal come within the field of atomic attraction of the other, forming nucleal bonds, the interfacial area of the metals being maintained in a protective atmosphere up to the squeezing of the metals, and heating or sintering the nucleally'bonded stock to effect lateral growth of the nucleal bonds to increase the strength of the bond. There is also disclosed in the copending application Serial No. 204,646 a method of determining what are referred to as characteristic bonding curves for any combination of metals, these curves being a plot of bonding temperature as against minimum percentage of reduction at which, for a given combination of metals, a nucleal bond capable of being converted to a good bond is obtained.

patented May 1, 1956 ice I have found that in solid phase bonding different metals of Widely different hardnesses at room temperature, and particularly in bonding soft malleable metals such as lead, tin, cadmium, zinc, gallium and their alloys to harder metals such as copper and steel by solid phase bonding processes such as are disclosed in the above-mentioned coassigned copending application patent, there may occur such a stretch-01f of the softer metal relative to the harder metal due to the intense pressure incurred in the bonding operation that the layer of the softer metal in the as-bonded (finally bonded) material is thinner than desired. This difficulty in general cannot be resolved by starting with a relatively thick layer of the softer metal that would be practical. 7

One object, therefore, of the invention is the provision of a method of solid phase bonding of different metals of normally widely different hardnesses in such a way as to reduce the stretch-off of the softer metal relative to the harder metal, thereby making possible the production of composite stock in which the thickness of the softer metal relative to that of the harder metal in the as-bonded stock is considerably increased over that obtainable by the processes disclosed in the above-mentioned applications, without requiring starting with an impractically thick layer of the softer metal.

In addition to the above, in the case of the soft metals mentioned above, the roll pressure may not be transmitted to the interface between the metals being bonded in a sufficient degree to provide the atom-to-atom contact to the extent desirable for obtaining a good bond. Another object, therefore, of the invention is the provision of a method of solid phase bonding of soft metals to metals of normally much greater hardness, in such a way that the bonding force is transmitted effectively to the interface of the metals being bonded together, and in such a way as to set up greater stresses and shearing forces at the interface to increase the bonding effect.

I have found that it is possible to accomplish the above objects by refrigerating the soft metal to increase its elfective hardness above normal, and squeezing the soft metal and the hard metal together with concomitant increase in the area thereof with the soft metal so refrigerated, the squeezing being suflicient at least to create nucleal bonds between the metals, these nucleal bonds being of the same character as described in said copending application patent. I have found that despite the refrigerated condition of the soft metals, the metals may be effectively solid phase bonded; and the refrigeration considerably reduces the stretch-01f of the softer metal so that a considerably thicker layer of the soft metal may be procured in the as-bonded stock than could be procured by bonding, fortexample, with the metals at or above normal room temperature, and increases the bond strength.

More particularly, I may proceed, for example, by first meticulously preparingthe surfaces of the metals to be bonded, to eliminate any films which would otherwise act as a barrier to solid phase bonding. This may be carried out in the same manner as described in the above-mentioned Patent 2,691,815, and reference is made to that patent for complete details. The softer metal is then refrigerated to increase its hardnes above normal. As-

suming that the metals are in the form of continuous strips,- and are to be bonded by rolling in a rolling mill, the soft metal strip may be refrigerated, for example, by passing it through a refrigerator wherein there is a cold, dry atmosphere, so dry as to eliminate any possibility of the formation of moisture on the metal. Then the metals are squeezed together in the rolls of the mill with the softer metal in its refrigerated condition, the squeezing effecting a considerable increase in the area of the mating surfaces of the metals, and being sufiicient to create nucleal bonds, in the same manner as disclosed in said Patent 2,691,815. Precaution is taken to preclude the formation of moisture at the interface of the metals up to the rolling operation. Following the squeezing operation, the nucleally bonded stock is subjected to a heat treatment adapted to cause the nucleal bonds to enlarge and increase the overall bond strength. This sintering heat treatment may consist simply of allowing the nucleally bonded material to warm up to room temperature. This is particularly so in the case of such metals as lead, tin, cadmium, zinc, and some alloys thereof which recrystallize at or slightly below room temperature. Or the material may be positively heated to a sintering temperature higher than room temperature. This heat treatment is analogous to the heat treatment disclosed in Patent 2,691,815, except that in general the sintering temperature will be much lower than the sintering temperatures therein disclosed.

The invention is applicable to bonding metals other than by rolling. For example, it may be applied to wire or tube drawing where a tube is to be bonded around a i rod or another tube by drawing through a die. It is applicable to extrusion of two metals jointly through an orifice of an extrusion press, and to extrusion by impact, as in forming a multilayered cup or tube. It is further applicable to upsetting operations where two rods are forced together endwise to form an area larger than the cross sectional area of the rod.

In each case an important feature is to squeeze the metals together with concomitant increase in area of their mating surfaces with a deformation sufiicient to create a solid phase bond, with at least the normally softer of the metals being refrigerated to increase its hardness.

As illustrative of the invention, I have bonded a short strip of lead 1% inches wide and .048 inch thick to a short strip of OFHC copper (soft) 1% inches wide and .056 inch thick by preparing the mating surfaces of the strips to remove barrier film, placing the strips together, and sealing them all around the edge to keep foreign matter from penetrating between them. Then I placed the assembled strips in a bath of solidified carbon dioxide and denatured alcohol having a temperature of from --90 F. to -100 F., leaving them in the bath long enough for the lead to cool to said temperature. Then I rolled the strips together in a rolling mill with such a reduction that the thickness of the lead was reduced 69% and the thickness of the copper was reduced 64%. This created a fair nucleal bond which, upon sintering at 300 F., was converted to an excellent bond.

Further illustrative of the invention, I have bonded a short strip of tin 1% inches wide and .048 inch thick to a strip of OFHC copper (soft) 1% inches wide and .056 inch thick in exactly the same manner as above, except that rolling was effected with a resultant 71% reduction of the thickness of the tin and a 57% reduction of the copper. The nucleal bond was good and the sintering at 300 F. converted this to a very good bond.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In the solid phase bonding of a pair of different malleable metals of substantially different hardness at normal room temperature wherein the softer metal hardcns substantially as temperature is reduced, squeezing the metals together and concomitantly effecting an increase in the area of the mating surfaces thereof to such an extent as to bond the metals together with the softer metal refrigerated below normal room temperature sufiiciently that its hardness under refrigeration is increased to a value substantially greater than its hardness value at said normal room temperature and substantially nearer to that of the other metal.

2. The process of solid phase bonding of a pair of different malleable metals of substantially different hardness at normal room temperature wherein the softer metal hardens substantially as temperature is reduced comprising refrigerating the softer metal sufficiently that its hard ness under refrigeration is increased to a value substantially greater than its hardness value at said normal room temperature and substantially nearer to that of the other metal, squeezing the metals together and concomitantly efiecting an increase in the area of mating surfaces thereof to such an extent as to bond the metals together, and subjecting the resultant stock to a heat treatment to improve the bond.

3. The process set forth in claim 1 wherein the softer metal is refrigerated to a temperature of the order of 100 F. and said softer metal is selected from the group consisting of lead, tin, cadmium, zinc and gallium.

4. The process set forth in claim 2 wherein the softer metal is refrigerated to a temperature of the order of -100 F., and said softer metal is selected from the group consisting of lead, tin, cadmium, zinc and gallium, and wherein the heat treatment involves heating of the metals .to a temperature above the recrystallization temperature of the softer metal.

References Cited in the file of this patent UNITED STATES PATENTS 473,884 Lipe et al Apr. 26, 1892 1,653,378 Steel Dec. 20, 1927 2,409,422 Egan Oct. 15, 1946 2,428,518 Everett Oct. 7, 1947 OTHER REFERENCES Symposium on Effect of Temperature on the Properties of Metals, page 664, published 1932 by American Society of Mechanical Engineers.

Transactions of the Institute of Welding, pp. 163478, especially pp. 177, 178; November, 1945. 

1. IN THE SOLID PHASE BONDING OF A PAIR OF DIFFERENT MALLEABLE METALS OF SUBSTANTIALLY DIFFERENT HARDNESS AT NORMAL ROOM TEMPERATURE WHEREIN THE SOFTER METAL HARDENS SUBSTANTIALLY AS TEMPERATURE IS REDUCED, SQUEEZING THE METALS TOGETHER AND CONCOMITANTLY EFFECTING AN INCREASE IN THE AREA OF THE MATING SURFACES THEREOF TO SUCH AN EXTENT AS TO BOND THE METALS TOGETHER WITH THE SOFTER METAL REFRIGERATED BELOW NORMAL ROOM TEMPERATURE SUFFICIENTLY THAT ITS HARDNESS UNDER REFRIGERATION IS INCREASED TO A VALUE SUBSTANTIALLY GREATER THAN ITS HARDNESS VALUE AT SAID NORMAL ROOM TEMPERATURE AND SUBSTANTIALLY NEARER TO THAT OF THE OTHER METAL. 